Genetics: Mendelian Genetics (2) Patterns of Inheritance. Outline. Incomplete dominance. 6. Extending Medelian Genetics

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1 Genetics: Mendelian Genetics (2) Patterns of Inheritance Outline 6. Extending Medelian Genetics a. Incomplete Dominance b. Codominance c. Polygenic Inheritance d. Environmental Impact 7. Mendilian Inheritance in Humans 8. Key Terms 9. Conclusions Incomplete dominance Incomplete dominance heterozygous phenotype is intermediate between two parents for example, snapdragons: P: Red x White (RR x R R ) F1:Pink (RR ) F2: ¼ Red(RR), 2/4 Pink(RR ), ¼ White(R R ) 1

2 Incomplete dominance Incomplete dominance Codominance Human Blood types The gene located on chromosome 9 Three alleles I A, I B, i (4 types: A, B, AB, O) Based on markers on red blood cells [Blood Type] 1. A (I A I A, I A i) anti B antibodies 2. B (I B I B, I B i) anti A antibodies 3. AB (I A I B ) none 4. O (ii) anti A and anti B antibodies 2

3 Polygenic inheritance Polygenic inheritance when alleles at more than one locus contribute to the same trait for example: (1) skin color (2) human height Polygenic inheritance 3

4 Environmental Impact Mendilian Inheritance in Humans 1. Pedigree analysis 2. Recessive inherited diseases 3. Dominant inherited diseases 4. New tools for genetic testing Pedigree analysis 4

5 Pedigree analysis Patterns of Autosomal Recessive Inheritance Both Parents are carriers Sickle cell anemia Sickle cell anemia a recessive disease; one amino acid changes in hemoglobin molecule; Sickle-cell hemoglobin molecules tend to cluster together and block the capillary vessels. normal SS, disease ss, carriers Ss. In Africa, 45% of certain population have the genotype Ss. Because the heterozygotes (Ss) are resistant to the disease Malaria (a deadly disease). 5

6 Sickle cell anemia Pleiotropic effects One gene has Multiple phenotypic effects Cystic Fibrosis 4% whites are carriers the most common lethal genetic diseases in the US. Most children with cystic fibrosis die before five years old. Now may live to over 20s. The gene can hide in the population for long time. 0.1% X 0.1% it is still there Recessive disease Parents: Normal heterozygous for cystic fibrosis P: Cc x Cc [Punnett Square] \ C c C CC Cc c Cc cc 25% normal, 50% carriers, 25% with the disease Patterns of Autosomal Dominant Inheritance 1. Achondroplasia 1/10, Huntington disorder no symptoms until the carrier is about 35 to 45 years old disease gene located on chromosome # 4 6

7 Patterns of Autosomal Dominant Inheritance One type of dwarfism in humans is caused by a single dominant gene; the condition is called Achondroplastic dwarfism. Dwarf individuals are heterozygous, while persons who are homozygous recessive are of normal stature. The homozygous dominant individuals all die before birth. (20% family history, and 80% new mutations) If two achondroplastic dwarfs marry and have a family, what fraction of their living offspring will also be achondroplastic dwarfs? Dwarf Aa Normal aa AA die P: Aa X Aa F1 genotypes: AA(die)Aa Aa aa 2/3 to be dwarf Character Trait True-breeding Hybridization Monohybrid cross Dihybrid cross Alleles Dominant allele Recessive allele Homozygous Heterozygous Phenotype Genotype Law of segregation Law of independent assortment Incomplete dominance Codominance Polygenic inheritance Pleiotropy Carrier Cystic fibrosis Sickle-cell disease Achondroplasia Huntington disorder In Conclusion 1. A gene is a unit of information about a heritable trait 2. Mendel provided evidence of dominant and recessive genes 3. Monohybrid crosses are crosses between two individuals that are homozygous for different versions of a trait 4. Crosses from F 1 result in F 2 offspring with phenotypes having a 3:1 ratio 5. Dihybrid crosses result in 9:3:3:1 phenotypic ratio 7

8 In Conclusion 6. Theory of Independent Assortment states that gene pairs independently sort out into different gametes regardless of other gene pairs of other chromosomes 7. Gene dominance can be in different level from complete dominance to incomplete dominance to co-dominance 8